The present invention relates to the manufacture of fiber-reinforced ceramic matrix composite articles, and more particularly to improvements in the process for manufacturing such composites with thermoplastic binder materials.
Ceramic matrix composites are well known. These composites typically comprise a ceramic matrix material such as a glass, a glass-ceramic, or a crystalline ceramic in which is disposed a reinforcing phase consisting of a multiplicity of refractory inorganic fibers. U.S. Pat. Nos. 4,314,852, 4,588,699 and 4,615,987 disclose examples of these materials and methods for their manufacture.
Conventional manufacturing processes for these composites have comprised, first, preparing a flowable suspension the ceramic matrix material in powdered form, coating or immersing the inorganic fibers (usually long fiber tows) with the suspension, and collecting the coated fiber tows, frequently into a mat or other shape. The impregnated fibers and shapes made therefrom are at this stage of the process commonly termed "prepreg". The prepreg thus provided can be reshaped as desired but is ultimately formed into a preform for the desired composite article, the preform is then subjected to a burn-out step wherein organic or other fugitive components present in the coating are removed from the preform, and the debindered preform is finally consolidated into a dense composite material by the application of heat and pressure.
Two different types of vehicle systems have been used for making the dispersions of ceramic matrix powders used in this process. The first type generally comprises substantial proportions of a solvent such as water or an organic liquid, with added organic binders to bind the powders together into a durable coating after the solvent has been removed by evaporation.
An alternative and more recently developed vehicle system, first disclosed in U.S. Pat. No. 5,024,978, utilizes a thermoplastic vehicle to form the ceramic powder suspension. Vehicles of this type are normally solvent-free, consisting almost entirely of low melting volatile waxes and organic polymer binders compatible therewith. Powder dispersions formed with these vehicles are room temperature solids which must be heated to reach the fluidity need for coating the fibers.
Prepreg formed with powder dispersions of the latter type does not require drying, since the vehicle solidifies as soon as the coated fibers are cooled. However, as a consequence of the fact that no solvents or other materials are removed during the course of prepreg and preform shaping, the resulting preforms incorporate higher proportions of organics at the burnout stage than prepreg formed with solvent-based ceramic powder suspensions. Thus prepreg with an organic component comprising 60 percent or more by volume is not uncommon using this process.
One consequence of this high binder content is that softening and extensive flow of the thermoplastic binder can occur during binder burnout. This can cause migration of the matrix powder within the preform, producing matrix-depleted and highly matrix-enriched regions within the resulting composite ceramic product. In addition, preform shrinkage between burnout and final consolidation is very high, increasing the difficulty of making complex shapes or achieving precise dimensions in the composite ceramic product.
Another difficulty with thermoplastic binder systems relates to prepreg instability during storage. Since volatile waxes constitute a major part of these binders, moderate storage temperatures must be maintained in order to avoid prepreg sticking. And, wax migration during storage or during collection of the prepreg after fiber coating can result in less-than-optimum textile quality in preforms formed by laying up or weaving the coated fiber tows into mats, tapes, or fabrics.
It is therefore a principal object of the present invention, to provide an improved process for the manufacture of ceramic matrix composites utilizing thermoplastic binders which reduces the sensitivity of the prepreg material to temperature variation in storage, reduces binder flow during burn-out, and improves the textile quality of prepreg materials provided from thermoplastic binder systems.
Other objects and advantages of the invention will become apparent from the following description thereof.